Many enzymes are now known to be subject to a covalent modification mediated by mixed-function oxidation. This modification may have diverse physiologic and pathologic significance. This includes the regulation of protein turnover, accumulation of modified proteins during aging, killing of pathogens by host defense mechanisms, limitation of autolysis, and oxygen toxicity. The role of oxidative modification in marking proteins for turnover is supported by the purification of a protease from E. coli which specifically degrades oxidatively modified glutamine synthetase. The unmodified protein is not a substrate. Multiple oxidative modifications may be introduced into a protein. Glutamine synthetase was subjected to varying times of exposure to mixed-function oxidation to provide samples of graded oxidation. The enzyme lost catalytic activity due to oxidation of a specific histidine residue. However, susceptibility to proteolysis did not correlate with this oxidation. Following a lag period, a second histidine residue was oxidized. The time course of this modification did match that of proteolytic susceptibility. The enzyme underwent other changes during modification, including exposure of sulfhydryl residues, weakening of subunit interaction, formation of aggregates, and decreased thermal stability.